Improvement of transparent conductive properties of Cu films by introducing H₂ into deposition atmosphere during RF magnetron sputtering

Highlights

• Cu films with different thicknesses are prepared in Ar and Ar + H₂ atmospheres.

• The ρ, n, μ and R_s of Cu films with different thicknesses are investigated.

• High FOM of Cu films can be obtained by optimizing the film thickness.

• Effect of introducing H₂ on structure and properties of Cu films is analyzed.

• Transparent conductive properties of Cu films can be improved by introducing H₂.

Abstract

Different thicknesses (2–250 nm) of Cu films were deposited on glass substrate by RF magnetron sputtering in Ar and Ar + H₂ atmospheres at room temperature. It is found that crystallinity and conductive properties of the films enhance but visible light transmittance (T_Vis) decreases with increase of film thickness. Compared with the Cu films deposited in Ar, those deposited in Ar + H₂ have higher crystallinity and surface roughness, and they have better conductive properties and higher T_Vis at film thickness above 5 nm. Overall, the Cu films deposited in Ar and Ar + H₂ can be achieved higher figure of merit (FOM) in a thickness range of 3~5 and 5~8.5 nm, respectively. Furthermore, the higher FOM values of Cu film deposited in Ar + H₂ atmosphere are higher than those of Cu film deposited in Ar atmosphere. The correlation between the improvement of transparent conductive properties of Cu films and modification of film microstructure due to introduction of H₂ into deposition atmosphere is discussed.

Introduction

Among all common metal materials, copper (Cu) is inexpensive, and its electrical conductivity is second only to Ag (1.67 μΩ cm). Because of high electrical conductivity, Cu films can be applied to electrodes, large scale integrated circuits and spin devices [[1], [2], [3]]. For ultrathin Cu films (≤10 nm), they have high transmittance and good electrical conductivity at the same time. Therefore, ultrathin Cu films and oxide/Cu/oxide tri-layer films based on ultrathin Cu film are promising alternatives to indium tin oxide (ITO) films as the transparent electrodes in light emitting diodes, solar cells and other photovoltaic devices [[4], [5], [6]].

The preparation methods of Cu films include sputtering [[1], [2], [3], 7], evaporation [[5], [6], 8], chemical vapor deposition [9], atomic layer deposition (ALD) [4, 10], and electroplating [11]. Among them, magnetron sputtering has been frequently used for deposition of Cu films. Usually, the Cu films are deposited in an Ar atmosphere, and the effects of sputtering power, pressure and substrate temperature on the microstructure and opt-electric properties of Cu films have been investigated [[12], [13], [14], [15], [16]]. Recently, Zhao et al. found that introducing suitable flux of O₂ or N₂ into the deposition atmosphere of Cu films could improve the transparent conductive properties of tri-layer films based on Cu films [17,18]. The reason behind this result can be attributed to the fact that O₂ or N₂ restricts the migration of Cu clusters on the substrate surface, which is favorable to the formation of continuous films at thinner film thickness. The Cu film can obtain high transmittance due to the thin film thickness, and at the same time, it can obtain highly electrical conductivity due to the formation of the continuous film. However, it is known that Cu is easy to react with O₂ and N₂, so the O₂ or N₂ flux needs to be carefully controlled during deposition of Cu films. Once the O₂ or N₂ flux is too high, the electrical conductivity of the Cu film decreases significantly. On the other hand, the introduction of O₂ or N₂ restricts the migration of Cu clusters on the substrate surface, thus reducing the crystallinity of the film [[17], [18], [19]]. The degradation of the crystallinity is not in favor of obtaining Cu films with highly transparent conductive properties. In view of the shortcomings of introducing O₂ or N₂ into deposition atmosphere, it is necessary to study the effects of introducing other gases on the structure and properties of Cu films, so as to provide more options for improving the transparent conductive properties of Cu film. Among other gases, the nature of H₂ is obviously different from that of O₂ (or N₂), and thus the effect of introduction of H₂ into the deposition atmosphere on growth and transparent conductive properties of Cu films should be different. Unfortunately, the effect of introducing H₂ on the structure and transparent conductive properties of Cu film has not yet been systematically studied.

In this study, the Cu films with different thicknesses (2–250 nm) were prepared by magnetron sputtering in 2 atm (Ar, Ar + H₂). The conductive properties (sheet resistance, resistivity, carrier concentration and mobility) of Cu films prepared in two different atmospheres as a function of film thickness were investigated. For ultrathin Cu films (2–10 nm) prepared in two different atmospheres, their transmittance properties were also studied as a function of film thickness. The results show that the introduction of H₂ into deposition atmosphere can obviously improve the transparent conductive properties of Cu films. By analyzing the XRD and AFM results of two kinds of Cu films, the improvement of transparent conductive properties of Cu films can be attributed to the improvement of film crystallinity by introducing H₂ into deposition atmosphere. This work provides a simple and feasible method to improve the transparent conductive properties of Cu films, and provides a reference for further development of Cu-based multilayer transparent conductive films.